This invention relates to a welding collar of fusible plastic with a heater coil disposed on its inside surface.
A welding collar of this kind is disclosed in U.S. Pat. No. 4,906,613 and contains a heating coil or heater winding of electrically conductive wire in the area of its inner surface. This prior art welding collar is configured as a double collar in which ends of plastic pipes to be joined are introduced from both sides. After introduction of the pipe ends into the welding collar, a current is fed to the heater winding so that the plastic in the interior of the welding collar and the plastic of the outer surface of each pipe end are heated to produce a welded connection. Such welding collars can be configured both as double collars and also as components of fittings or comparable components in pipe lines.
Plastic pipes, in practice, often do not have an ideal axially elongated form, particularly when they are supplied in coils and, accordingly, are curved. When pipes taken from coils and other goods to be connected are welded by heater coil welding, due to the great inherent tensions resulting from the curvature of the pipe, a reliable weld can be assured only by additional aids such as straightening, stretching and holding devices. Also, inherent tensions and the stiffnesses of the pipes become greater as the ambient temperature decreases. Handling of the pipes and working with the pipes in pipe trenches become especially difficult under such conditions. The length of the prior art double collar is not much greater than the inside diameter or nominal size of the double collar. If one considers only the portion of the collar for welding one pipe end, that is, half of the double collar, then the length of this partial welding collar is substantially smaller than the nominal size.
A welding collar avoiding additional holding devices is disclosed in Japanese Patent 07-145,899 A. The welding collar has, in its interior, internal centering ribs tapering conically and extending axially. In the middle of its length, this welding collar, configured as a double collar, contains an annular abutment for the pipe ends. The centering ribs, which permit the pipe end to be gripped, are disposed between the annular abutment and the particular heater coil which is disposed at an axial distance. With internal centering ribs of this kind, the danger exists that the pipe ends will not be inserted all the way to the abutment, because the guiding ribs can limit the insertion movement. The manufacturing tolerances for the welding collar including the centering rings must be tight, inasmuch as the manufacture of the pipes is performed with comparatively great tolerances for practical and economic reasons. If the pipe end is not introduced all the way, then there is the danger of a faulty welded joint.
Furthermore, German Offenlegungsschrift DE 44 22 372 A1 discloses a method for welding plastic pipes and a collar for practicing the method. The collar, again, is a double collar and contains a spiral type heater winding extending over the entire length of the collar. In the middle part of the collar, in which the two pipe ends to be joined abut one another, the windings of the heater coil are closer together than they are in the areas of the heater coil adjoining the collar ends. This is done for the purpose of joining the abutting ends directly with a butt weld. The pipe ends are held by a special holding tool. For pipes of small diameters, the double collar can have additional projections near the two collar ends in order to be able to dispense with the holding device. The axial length of the projection is substantially smaller than the axial length of the heater coil or welding zone.
In German Utility Model No. DE U 296 19 861, a plastic welding collar is configured as a double collar for connecting two pipe ends and has special clamping devices at its two ends. The plastic collar body has axial slits in the areas of the clamping devices so that radial forces can be exerted by the clamping devices to hold the pipe ends. The operation of the clamping devices requires additional assembly work. If the clamping is inadequate, then the collar may not be properly secured to the pipe end.
Moreover, Japanese application JP 07-145,897 A teaches a welding collar which has projections extending radially inward at both collar ends and contacting the outer surface of the pipe. The projections are made quite thin in comparison to the wall thickness of the collar body and thus have a slight radial stiffness. The projections are said to achieve an axial hold, but radial equalization movements are possible and centering of the pipe end in the collar cannot be achieved.